My ESPP Dilemma

I've been working for the company for ~ 8 years and have participated in the employee stock purchase plan since day one. It's been a great deal, the ability to purchase company stock at a discount and the appreciation of the stock. I have not sold any stock in the company. Now it makes up approximately 40% of my total portfolio. I've been able to generate an annual return of ~ 34% by just holding onto the stock. I'm tempted just to lighten up to diversify a bit. It would probably go into an index fund or real estate (if there's a good deal). It's very tempting to just hang on, but we all know what happened to the Enron...